Conventionally, semiconductor crystals are suitably used as a substrate for an optical device such as a light emitting diode (LED) and a laser diode (LD), or a substrate for an electronic device such as a transistor.
It is known that device properties of such an optical device and an electronic device are greatly affected by the impurity concentration, carrier concentration and the like of a substrate. Therefore, to solve variations in device properties among optical devices and electronic devices, it is necessary, for example, to make the impurity concentration distribution in a used semiconductor crystal more uniform.
FIG. 6 shows a schematic cross sectional view of an example of a conventional apparatus for producing a semiconductor crystal. This production apparatus 200 includes a crucible 20, a heat-resistant container 21 that encloses crucible 20, heaters 22a to 22d provided outside heat-resistant container 21, a B2O3 seal 23 for sealing heat-resistant container 21, and a chamber 24 that stores these members.
According to a method of producing a semiconductor crystal using this production apparatus 200, first, a GaAs seed crystal 25 is disposed at the bottom of crucible 20, and a raw material made of a GaAs polycrystal containing Si as an impurity and a sealant 26 made of B2O3 are placed sequentially on this GaAs seed crystal 25. Then, heater 22d heats B2O3 seal 23. B2O3 is thereby melted to seal heat-resistant container 21. Then, heaters 22a to 22c heat crucible 20, so that the GaAs polycrystal in crucible 20 is once melted.
Thereafter, the temperature of a GaAs melt 27 derived from the GaAs polycrystal is gradually decreased upward from the bottom of crucible 20. Along with this temperature change, GaAs melt 27 solidifies from its lower side that is in contact with seed crystal 25 located at the bottom of crucible 20 toward its upper side, so that a Si-containing GaAs crystal 28 is produced. GaAs crystal 28 extracted from crucible 20 is cut out in a direction orthogonal to its growth direction for use as a substrate for an optical device or a substrate for an electronic device.
However, Si in GaAs melt 27 has a segregation coefficient less than or equal to 1 relative to GaAs crystal 28, and is thus incorporated into GaAs crystal 28 only in a concentration lower than the Si concentration in GaAs melt 27 due to a segregation phenomenon. Therefore, the Si decrease amount in GaAs melt 27 is less than the amount of decrease of GaAs melt 27, so that the Si concentration in GaAs melt 27 increases as GaAs crystal 28 grows. Accordingly, the Si concentration in GaAs crystal 28 increases gradually from its lower side to its upper side, which causes variations in impurity concentration among a plurality of substrates cut out from a single GaAs crystal 28.
On the other hand, Japanese Patent Laying-Open No. 2005-350295 (Patent Literature 1) discloses an apparatus in which a raw material as a semiconductor polycrystal is placed in a container arranged above a crucible, as shown in FIG. 7. In a crystal production apparatus 300 of FIG. 7, heaters 31a to 31e each generate heat, thereby creating a temperature gradient in the vertical direction in the crystal production apparatus. In this crystal production apparatus 300, a crucible 32 is heated with heaters 31a to 31c, so that a GaAs polycrystal in crucible 32 is melted. Then, the temperature of a GaAs melt 33 containing Si derived from the GaAs polycrystal decreases gradually from the lower side toward the upper side of crucible 32, so that Si-containing GaAs melt 33 placed in crucible 32 solidifies from the side (lower side) that is in contact with a seed crystal 34 to the upper side. On the other hand, simultaneously with the solidification of GaAs melt 33, a raw material 36 placed in a container 35 is melted from the lower side toward the upper side of container 35. Since a melt 37 derived from raw material 36 is dropped into Si-containing GaAs melt 33 located under a sealant 40, changes in Si concentration in the melt in the crucible along with crystal growth can be compensated for, as a result of which a GaAs crystal 38 having a uniform carrier concentration can be produced.
Japanese Patent Laying-Open No. 9-52788 (Patent Literature 2) discloses an apparatus in which a raw material is charged into a tapered-shape container arranged above a crucible. Also in this apparatus, a semiconductor polycrystal is melted from the lower side toward the upper side of the container simultaneously with crystal growth from the lower side toward the upper side of the crucible. Therefore, similarly to the apparatus disclosed in Patent Literature 1, changes in Si concentration in the melt in the crucible along with crystal growth can be compensated for by dropping of a melt of the raw material in the crucible.